This invention relates to a method of imparting a property to a layer, the property being that liquid crystal molecules which may be placed on the layer would adopt a preferred alignment. The invention also relates to LCD elements incorporating a preferred alignment.
The operation of liquid crystal devices (e.g. liquid crystal displays and light valves and liquid crystal polymer elements such as optical retarders, polarisers, cholesteric filters etc.) requires controlled alignment and usually also pretilt of the liquid crystals. Currently, a mechanical rubbing technique is used to prepare surfaces which are capable of inducing alignment and pretilt.
To overcome the disadvantages of the rubbing technique, several optical methods have been developed, which use linearly polarised light and are generally called photo-alignment methods. These are disclosed in U.S. Pat. No. 4,974,941 Gibbons et al, U.S. Pat. No. 5,784,139 Chigrinov et al, U.S. Pat. No. 5,389,698 Chigrinov et al, and European Patent 0525478B (Hoffmann-La Roche et al).
While satisfactory in themselves, the methods disclosed in these patents rely on polarised light. Light sources producing polarised light are comparatively complex, which may be less suitable for mass production, and expensive. As a polariser usually absorbs at least 50% of the light, dispensing with the polariser would allow a much better usage of the light source (either a faster effect, or could use a weaker lamp). Therefore, already certain methods have been suggested, which use non-polarised light.
The generation of a pretilt angle in a nematic liquid crystal cell using a polyimide surface which has been irradiated with non-polarised U.V. light with an angle of incidence of 70xc2x0 to the normal to the surface has been disclosed by Seo et al in xe2x80x9cAsia Display 98xe2x80x9d paper P-81. pp 795-798 and in xe2x80x9cLiquid Crystalsxe2x80x9d, 1997 vol 23 no. 6 pp 923-925. However, this method does not benefit from the potential advantages which we have identified, requiring instead very high energy input, sufficient to depolymerise the polyimide.
We have found that under certain unexpected circumstances the optical photo-alignment method referred to above also works with light that is non-linearly polarised (e.g. circularly polarised) or isotropic (unpolarised).
According to the present invention there is provided a method of making an element of a liquid crystal polymer, comprising imparting a property to a material, said property being that cross-linkable monomeric or pre-polymeric liquid crystal molecules placed on a layer of the material or mixed with the material adopt a preferred alignment,
the method comprising exposing the material to unpolarised or circularly polarised radiation from an oblique direction, allowing monomeric or pre-polymeric liquid crystal molecules applied to or mixed with the exposed material to adopt the preferred alignment and while aligned cross-linking them.
Preferably, the angle of incidence xcfx86 of the radiation to the normal to the layer is within the range 5xc2x0xe2x89xa6xcfx86 less than 70xc2x0, more preferably exceeds 45xc2x0.
The radiation may be ultraviolet.
The said preferred alignment is preferably such that the longitudinal axis of the liquid crystal molecules is in the plane including the normal to the layer and the direction of the radiation. The alignment may be planar (0xc2x0) or tilted (up to 90xc2x0). The imparted preferred tilt preferably exceeds 45xc2x0 to the plane of the layer, and more preferably exceeds 75xc2x0.
In addition, the effect of the irradiation on the material may be to cross-link it, thus also improving the stability of the material and its aligning properties.
As for the radiation to which the material is exposed, this may be zonewise patterned, for example by, interposing a microelement array, such as a microlens or microprism array or a suitable hologram element, between the source of the radiation and the material, so that, in said imparted property, the preferred alignment is zonewise patterned. Using such a microelement array further allows to generate locally different oblique radiation from a single radiation source even if the source itself radiates perpendicular to the material layer or the microelement array.
This method when using irradiation with unpolarised, preferably ultraviolet, light in a specific illumination geometry, and with a suitable alignment layer material causes the conversion of a layer, which is isotropic before the irradiation, into an anisotropic layer. Layer and method typically have the following characteristic features:
(a) After conversion the layer has an aligning effect on a monomeric or pre-polymeric liquid crystal material put on the layer.
(b) Simultaneously with the generation of anisotropy in the layer, a cross-linking also occurs, i.e. the generation of the alignment capability and the cross-linking are based on a bimolecular photo process, but the method according to the invention can also apply to monomolecular processes, typically using azo dyes.
Where, as desirable, the layer of material has a photo-alignment sensitivity better than 2 J/cm2, and the irradiation energy (measured normal to the radiation) can correspondingly be kept to less than 2 J/cm2, productivity is enhanced since exposure times can then be reduced to less than 10 minutes.
The layers are photo-structurable, i.e. azimuthal alignment and tilt angle can differ in different parts of the layer (e.g. by exposing through photo-masks, holographic imaging, imaging through microprisms, microlenses, and pixellated light switches such as micro-mirrors).
On the other hand, devices which are aligned uniformly over large areas may also be made by the method, especially LCP retarders and optical compensators for improving viewing angle of displays.
The layers can find use as alignment layers for liquid crystal devices such as displays; the displays can contain monomeric nematic, cholesteric or smectic (including chiral smectic C) liquid crystals. The operating mode may be transmission or reflection. In reflection both specular metallic or diffuse reflectors can be used, as well as reflectors made of cholesteric layers or polarisation converting optical elements (e.g. BEF foils).
The device substrate may be glass, plastic, a silicon chip, or anything else suitable.
Advantages of bypassing the necessity for polarised light include, apart from those already mentioned, general simplification of the method and making it more adaptable to mass production, and the ability to use microlens-, microprism- or similar arrays for the illumination leading to a structured alignment with only one irradiation step, not possible with polarised light.
The invention may be used in relation to Vertically Aligned Nematic (VAN) cells, wherein the liquid crystal displays have a tilt angle of 90xc2x0xe2x89xa7xcex8 greater than 75xc2x0 on both surfaces, or in relation to Hybrid Aligned Nematic (HAN) cells, where the tilt angle on one surface is 90xc2x0xe2x89xa7xcex81 greater than 75xc2x0 and on the other is xcex82xe2x89xa630xc2x0. Intermediate tilt angles on one or both surfaces may also have utility.
The material as such may be substantially homeotropically orienting. That is, the material may, be one which induces an (azimuthally unoriented) large tilt angle, not necessarily exactly 90xc2x0, but preferably exceeding 80xc2x0, more preferably exceeding 85xc2x0, to liquid crystal molecules thereon. Particularly where large tilt angles are required, it may be advantageous to start with a substantially homeotropically orienting material, which will need (besides the azimuthal alignment) only a small adjustment of the induced tilt angle to achieve the exact required tilt.
The materials used in the invention may be photopolymerisable polymers, such as those which are also used in the known photo-alignment methods, particularly linearly photopolymerisable polymers.
The materials used can include not only photopolymerisable polymers but also monomolecular aligning materials which are inherently unstable because the photo-alignment does not cross-link them; however this does not matter if a liquid crystal polymer layer is applied while the monomolecular material is photo-aligned, as the said liquid crystal polymer can itself be cross-linked (stabilised in its aligned position), whereafter the instability of the monomolecular material has no damaging effect.
The invention can also be applied to a polymerisable mixture which comprises (i) a liquid crystal monomer or pre-polymer having cross-linkable groups, and (ii) a photo-orientable monomer or oligomer or polymer. Such mixtures are described in UK Patent Application 9812636.0, the disclosure of which is incorporated herein by reference. Despite the distinct functions of the participating molecules, these mixtures are capable of being both oriented and cross-linked into a liquid crystal polymer. These mixtures are hence usable on the one hand as anisotropic layers in optical components or on the other hand, normally applied more thinly, as orientation layers.
It is understood that substance (i) may also be a liquid crystal polymer mixture, i.e. may contain two or more different liquid crystal molecule types. Equally, substance (ii) may be a mixture of photo-orientable molecules. Assuming that the cross-linkable liquid crystal substance (i) is present in an amount of 100 parts, the photo-orientable substance (ii) preferably is present in an amount of at least 0.1 part, more preferably of at least 1 part, most preferably of at least 10 parts. A preferred photo-orientable substance (ii) comprises molecules showing a cis-trans-isomerism, particularly azo dyes. Another preferred photo-orientable substance (ii) comprises a linearly photo-polymerisable polymer. Depending on the intended application, the cross-linkable liquid crystal substance (i) may have a nematic phase or cholesteric phase or a ferroelectric phase respectively. The substance(s) (i) is/are preferably acrylate or diacrylate. The mixture may further comprise chiral molecules or dye molecules or dichroic molecules or fluorescent molecules.
The invention extends to an element made from a liquid crystal polymer by a method as set forth above. Such an element may advantageously comprise a plurality of sequentially applied, aligned and cross-linked liquid crystal polymer layers (or mixtures, as set forth above).
The invention further extends to an optical device relying on a liquid crystal polymer whose properties are fixed, comprising an element (for example an optical element) as set forth above. Examples of such devices would include an orientation layer, an optical retarder, a polariser, a cholesteric filter, or an element for protecting a document against copying or alteration.